Conventional liquid crystal (LC) devices of this type generally comprise a liquid crystal layer of controlled thickness (i.e. cell gap) sandwiched between two substrates. Each substrate is transparent and coated with a transparent, electrically conductive coating on the side facing the liquid crystal layer to enable an electrical field to be applied to the layer. The substrates may be glass or a polymer substrate film. If the substrates are film then it may be possible to laminate the liquid crystal film to regular window glass panes on one or both sides by employing an adhesive sheet known as an interlayer. Such a combined LC film and glass laminate is known as a switchable window. Saint Gobain Vitrage sells a switchable window laminate under the brand name “Priva-Lite”.
The process of laminating a liquid crystal film between glass panes using one or more interlayer sheets subjects the film to pressure, elevated temperature, and vacuum. Difficulties can arise due to mismatched thermal expansion indices between the different materials. Furthermore, even after lamination, subsequent handling of the finished laminate can subject the LC layer to shear forces as the two glass panes (sandwiching the LC layer) flex, especially if the window is greater than 1 meter in any direction. In order for the film to withstand the lamination process and subsequent handling it is necessary for the LC layer to have a polymeric (or other) structure to support the liquid crystal.
There are several known polymer-LC structures. Each suffers from drawbacks. Some are fundamentally unsuitability for lamination in this type of structure, whereas those that can be laminated suffer from optical problems such as excessive haze or an excessively limited range of useful transparent viewing angles.
When considering such problems it is necessary to evaluate and balance the conflicting requirements for a given application. Thus, while it may be relatively trivial to reduce haze or increase viewing angles for a particular film by reducing the thickness of the polymer-LC film, this can have a direct impact on the ability of the film to block light when in the supposed blocking state. Ensuring an acceptable level of opacity in the blocking state may require a film of a thickness which, in the supposedly transparent state, is inherently hazy and has very limited viewing angles.
The requirement for clear, haze-free viewing over as extensive a range of viewing angles as possible, is extremely important for window applications. A person standing directly in front of a 2 m×2 m window will encounter a range of viewing angles depending on the viewer's distance from the window and on which part of the window is being looked through. If the viewer's eye is 2 meters away from the centre of the window, for example, the angle to the corner of the window is about 35 degrees; in other words, for such a window to be transparent at a distance of 2 meters, it requires a haze-free viewing angle of 35 degrees. For the same window to be transparent from a distance of 1 meter, however, increases the required haze-free viewing angle to about 55 degrees.
The main technologies of polymer-LC structure are:                1) Polymer Dispersed Liquid Crystal (PDLC) and Nematic Curvilinear Aligned Phase (NCAP);        2) Polymer Stabilised Cholesteric Texture (PSCT);        3) Polymer Dispersed Cholesteric Liquid Crystal (PDCLC); and        4) Fast switching, low voltage devices having aspects of the previous three.        
Briefly, PDLC or NCAP devices have a continuous polymer structure (typically making up 40-60% of the liquid crystal layer) having discrete cavities that contain liquid crystal. The liquid crystal (nematic type) is said to form droplets within a continuous polymer matrix.
PSCT devices also rely on liquid crystal material dispersed throughout a polymer, but in PSCT devices the polymer does not encapsulate the liquid crystal in discrete droplets; instead it provides a thin, fibrous polymer network that extends into and/or through a continuous liquid crystal layer.
Generally, PDCLC devices are bistable, colour reflective devices (unlike the PDLC/NCAP and PSCT devices which are selectively transmissive). PDCLC devices have a cholesteric liquid crystal material dispersed in relatively larger droplets within a polymer matrix, with the LC material being switchable between two bistable states—one being highly reflective of a narrow band of visible wavelengths, and the other being weakly scattering. By placing the PDCLC film in front of (say) a black substrate, pixels of the display can be switched between colour reflective and black states.
Each of the three families of technology results in devices having certain advantages and certain disadvantages. Each will be described in turn in the following section before finally describing devices for fast switching and low voltage applications that have aspects common to more than one of the three.
PDLC or NCAP Devices
Polymer Dispersed Liquid Crystal (PDLC) and Nematic Curvilinear Aligned Phase (NCAP) refer to two very similar technologies distinguished from one another by the techniques used to create the respective devices. However, for the purposes of the following discussion, the end result of each technology is effectively the same.
FIG. 1 (Prior Art) shows an example of a PDLC device. A pair of polyethylene terephthalate (PET) substrates 110, each having an indium tin oxide electrode 112 on its inner face, sandwich a polymer-dispersed liquid crystal structure 114. The structure 114 consists of a 20 μm thick film composed of a polymer matrix or structure in which droplets 116 of nematic liquid crystal material have been captured by phase separation during polymerization.
The primary advantage of PDLC and NCAP type films is that the polymer structure has been demonstrated to be compatible with glass lamination. Furthermore, PDLC and NCAP films are flexible, can be cut to size from a continuous roll of film (as the liquid crystal is encapsulated by the polymer structure), and function without the need for polarizers (inherently required by twisted nematic (TN) type devices).
Referring back to FIG. 1, the liquid crystal droplets are generally spherical and have a diameter of about 0.7 μm to 1.0 μm. FIGS. 2A and 2B illustrate that in the presence of a 100V AC field (FIG. 2A) the liquid crystal 116 displays birefringence, with the refractive index of the polymer 114 matching that of the liquid crystal in the direction parallel to the major axis of the film (i.e., the ordinary refractive index, no), while in the absence of such a field (FIG. 2B), the mean refractive index of the LC material is mismatched with that of the polymer (npolymer≠neffective LC), resulting in scattering at each boundary between liquid crystal and polymer crossed by a light ray passing through the film.
The major problem with PDLC and NCAP devices in window glazing and see-through applications is they suffer from significant haze in the ON state as the viewing angle increases, and can become opaque for large viewing angles. The haze is caused by light scattering at the boundary or interface between the field-aligned, nematic, liquid crystal in the droplets and the encapsulating polymer. A mismatch between the refractive index of the polymer and that of the liquid crystal in the droplets due to its inherent birefringence is the reason. For example, the material sold by Merck under the catalog number “Merck TL213” has ordinary and extraordinary refractive indices of no=1.527 and ne=1.766, respectively, giving a birefringence value of Δn=0.239. Generally, the ordinary refractive index of the liquid crystal is matched to the refractive index of the isotropic polymer matrix to minimize haze. But, as the viewing angle increases in the ON state the significance of the mismatch between the polymer and the liquid crystal becomes more dominant (npolymer≠neffective L.C.) and light scattering at the interface increases causing haze.
Another drawback of PDLC and NCAP devices is that in the OFF state (no electrical field) scattering light efficiency is determined by the difference between the mean refractive index of the liquid crystal and the refractive index of the polymer matrix. It follows from this that interface surface must be maximized by minimizing droplet size and maximizing the number of droplets. The minimum size is dictated by the wavelengths of visible light, and so PDLC and NCAP displays typically have a mean droplet diameter of 0.7 μm-1 μm (this refers to the major axis as the droplets may not be spherical).
In a typical 20 micron PDLC cell, a light ray could encounter 10 droplets or more, each having the potential to contribute to haze in the ON state, even for viewing angles close to normal. This demonstrates the inherent trade-off in PDLC devices whereby increasing the scattering power in the OFF state increases the haze in the ON state.
U.S. Pat. No. 5,604,612 discloses a PDLC device of this type, and discusses how scattering power can be maximized by optimizing the difference between the mean refractive index of the liquid crystal and the polymer matrix.
PSCT Devices
An example of a normal-mode Polymer Stabilised Cholesteric Texture (PSCT) film is shown in FIG. 3. For ease of illustration, the two operating states of interest—homeotropic (transparent) and focal conic (scattering)—are shown side by side.
The polymer in this device does not encapsulate the liquid crystal in discrete droplets, but rather it provides a thin, fibrous polymer network 120 that extends into and/or through a continuous liquid crystal layer 122 as shown in FIG. 3. It has been shown that the polymer is effective in separating the continuous liquid crystal layer into domains that individually switch quicker than a continuous layer without polymer. The liquid crystal is of the type known as cholesteric or chiral nematic. Because the LC layer is continuous, the film must be sealed using a seal such as that shown schematically at 129.
In PSCT devices a chiral dopant is added to nematic liquid crystal to impart an alignment between molecules 127, see FIG. 4, each at a slight angle to the next, which traces out a helical structure. The distance required for one full twist of the helix is known as the pitch. The pitch can be adjusted by adjusting the concentration of the chiral dopant. By selecting the pitch length accordingly, the planar texture reflects a band of visible light (this type of device is known as reflective); but when the pitch is increased further the reflected light will move into the infrared range, and light in the visible range is transmitted (this type of device is known either as normal or reverse mode).
In liquid crystal domains containing planar texture, the axes of the helices align parallel to one another and perpendicular to the substrate surface. This texture selectively reflects circularly polarized light for a band of wavelengths associated with the helical pitch of the cholesteric. Normal mode light shutters do not use the planar texture. Reverse mode light shutters use it as the transparent or clear state and avoid the homeotropic state.
The focal conic texture is similar to the planar texture in that the chiral dopant imposes a helical alignment between liquid crystal molecules, but unlike the planar texture, the axes of the helices align poorly with respect to each other—see texture on right hand side 126 of FIG. 3. Poor alignment creates an angular difference that results in the effective refractive index of the liquid crystal in one helix (or in one domain containing aligned helices) being different from its neighbouring helices (or domains) thereby causing light scattering at the boundaries. Consequently, PSCT devices have a different scattering mechanism, namely the focal conic texture, to that of PDLC devices.
In addition to the switching benefits of separating the liquid crystal into domains, a polymer network also exerts a stabilizing influence whereby the liquid crystal molecules adjacent the polymer interface take on an alignment. This alignment may be sufficient to stabilize a cholesteric liquid crystal domain in one or more of its three possible states (or textures) in the absence of power: planar (light reflecting and transmitting), focal conic (light scattering and/or transmitting), and homeotropic (clear or transparent).
In normal mode PSCT devices, the principal function of the polymer is to stabilize the focal-conic texture. The more fibrous the polymer network, the more effective it is in inducing random alignment of the helical axes (i.e. creating multiple domains) in the focal conic texture, and consequently a strong scattering state that blocks visual access through the PSCT film. It is important to note that in the absence of a polymer network, or other means such as polymer surface artefacts, to stabilize the focal conic texture, then on removing power random helical alignment will not persist, i.e. such a texture is not stable over time. In this case the focal conic texture will revert to a weak scattering/transmitting state typical of reflective PSCT devices.
The homeotropic texture is the only texture that is common to both PSCT and PDLC devices. In the presence of a strong electrical field the helices unwind in PSCT films and the liquid crystal director (i.e., the common direction of the long axes of the liquid crystal molecules) aligns parallel to the field (assumes positive dielectric anisotropy)—see texture on left hand side of FIG. 3.
U.S. Pat. No. 5,437,811 teaches normal (opaque in the absence of power) and reverse-mode (transparent in the absence of power) PSCT light-shutters that are virtually haze-free regardless of viewing angle, and have superior optical clarity to PDLC and NCAP displays even when viewing normal to the display. While the polymer percentage of the liquid crystal layer can be up to 40%, and the polymer type can be isotropic or mesogenic (i.e., a Liquid Crystal Polymer LCP), generally such devices will only exhibit good optical clarity when the polymer percentage is <10%. In addition, an electrical or magnetic field must be present during curing, and this is undesirable in a continuous film manufacturing line.
U.S. Pat. No. 6,049,366 teaches one method to manufacture PSCT light shutters for switchable window applications (referred to more generally as Polymer Stabilized Liquid Crystal PSLC in the document) on large flexible film, including the steps of providing a replicated polymer structure within the liquid crystal layer. The latter is a relatively complex process.
In U.S. Pat. No. 6,671,008 the PSCT material is filled directly between large glass panes to realise a window without the conventional steps of first producing a liquid crystal film, then laminating the film to glass panes as discussed earlier. While the disclosed method may seem attractive, there is complexity in coating standard window glass with a transparent conductor, and in getting a flatness and finish quality compatible with liquid crystal. Other manufacturing issues include filling a large area uniformly with a liquid crystal polymer mixture, achieving sufficient strength in an edge seal area to withstand the shear forces under glass flexing for X or Y dimensions >1 meter, and not least, coping with a glazing industry than is not characterized by standard sizes.
U.S. Pat. No. 7,023,600 discloses a method to make bistable, PSCT films for switchable-window applications, whereby selectively the focal conic (strong light scattering) or planar (visible light transmitting—transparent) states are stable in the absence of power. The disclosed switchable window film has the advantage that power is used only when switching from one state to the other, and no power is consumed to maintain the window in either the planar/transparent or strongly-scattering, focal conic states. While this is particularly attractive for battery-powered applications, the disclosed bistable device requires relatively high frequency (>1 KHz) switching when changing from clear to opaque, and the feasibility of applying high frequency switching to large-area (i.e., >1M2) light shutters having film substrates needs to be demonstrated.
Unlike PDLC devices where the polymer structure bridges both substrates in a continuous layer, the polymer structure in PSCT cells, whether film or glass substrate, is only directed to bridging both substrates when polymer network is formed from substantially mesogenic monomer in the presence of an electrical field. In the absence of an electrical field, or for isotropic monomers, a substantial part of the polymer will form on the surface of the substrates, particularly the substrate facing the ultra-violet light curing source, resulting in a polymer layer that contributes very little to film structure. If the polymer content (and/or monomer functionality) is increased to force more bridging, then the optical clarity suffers greatly as found in U.S. Pat. No. 6,049,366 for PSCT examples having >20% polymer.
In normal-mode PSCT films containing mixtures of mesogenic and/or isotropic monomers, the polymer networks are prone to damage resulting in unacceptable localized optical degradation. Other problems include network damage from bending of the film, especially when bending from a corner, regions of broken network and non-uniform cell thickness resulting from laminating the film to glass, and broken network resulting from picking up a large glass laminate by a corner, or flexing the glass.
In summary, PSCT devices, whether produced on film or glass substrates, are characterized by relatively little increase in haze with viewing angle when compared to PDLC devices. This accrues from having the liquid crystal in a continuous layer. Optical clarity is best when the polymer content, present in the liquid crystal region as polymer network, is <10%. PSCT devices have an alternative scattering mechanism to PDLC devices, focal conic texture, but require a polymer network to stabilize the focal conic texture with sufficient scattering power to block visual access. However, despite having superior optical characteristics to PDLC devices, PSCT films made according to prior art methods have insufficient mechanical strength to be suitable for the demands of the applications contemplated herein.
PDCLC Devices
Polymer Dispersed Cholesteric Liquid Crystal PDCLC devices from the prior art are designed for reflective display applications and do not work as switchable windows. An example of a reflective, bistable PDCLC film is shown in FIG. 5, which again shows the two states alongside one another, in this case the reflective planar texture on the left, as indicated at 130, and the weakly scattering focal conic texture on the right, as indicated at 132. The film again has a pair of substrates 110 carrying electrodes 112, which sandwich a polymer structure 114. In this case the liquid crystal is provided as larger volumes 116, as explained further below.
When incident light 133 falls on the planar texture, a band of circularly polarized light is reflected 134 while the remaining light is transmitted by the liquid crystal layer but absorbed by black paint 136 on the rear substrate. The focal conic texture transmits light, weakly scattered, to the light absorbing (i.e. black paint) coating on the rear substrate. The homeotropic state is not used in this type of display as it is not stable in the absence of power, unlike the other two states that are. Exemplary reflective displays are discussed in the January 2007, Journal of the Society For Information Display (SID), “Progress in flexible and drapable reflective cholesteric displays”, and shown on Kent Displays, Inc. website: www.kentdisplays.com
Unlike PSCT devices, PDCLC devices do not rely on a polymer network to stabilize the liquid crystal textures, rather the anchoring of the liquid crystal molecules to the polymer surface is sufficiently strong and uniform to induce the planar state. The focal conic state only weakly scatters light because the same polymer surface anchoring which allows for a stable planar texture also imposes a strong ordering within the focal conic texture. This can be seen by comparing the weakly scattering focal conic texture of FIG. 5 with the strongly scattering texture of FIG. 3. For reflective PDCLC applications, weak scattering is highly desirable, but it makes such devices unsuitable for switchable windows and similar applications.
PDLC type displays are compared to reflective PDCLC displays in the article titled “Flexible Encapsulated Cholesteric LCDs by Polymerization Induced Phase Separation”, by Tod Schneider et al. in the Society for Information Display SID 05 Digest, pages 1568-1571. The article is based on disclosures in US Patent Publication No. 2007/0026163. “In a typical PDLC, the droplets are generally spherical, less than 1 micron in diameter, and are numerous in number throughout the thickness of the cell. In [the article's PDCLC displays] the droplets are more pancake in shape, on the order of magnitude of ˜10 micron (or more) in diameter, and are singular throughout the thickness of the cell. [The article's PDCLC displays] operate in the reflective and scattering (to a light-absorbing back plane) modes, i.e., [they] have very little light scattering in both modes”.
In “Anchoring Behaviour of Chiral Liquid Crystal at Polymer Surface: In Polymer Dispersed Chiral Liquid Crystal Films”, by Haixia Wu, Georgia Institute of Technology, 2004 the chemical structure of acrylate and methacrylate monomers (i.e., prepolymers) most likely to promote strong uniform anchoring to stabilize the planar state for bistable, reflective PDCLC displays are discussed. In the PDCLC trials conducted the droplet shape was described as polygonal having X-Y axis in the range 30-50 micron, and a cell gap of 10 micron. However, the area found to be fully functional within these polygonal droplets was only around 4 μm2, and the polymer content of the liquid crystal layer was only 10%.
In U.S. Pat. No. 6,061,107 polymer/cholesteric liquid crystal dispersions are provided in which, similar to PDLC devices, the liquid crystal phase separates into discrete droplets within a continuous polymer matrix. But, unlike PDLC displays where the droplets have a major axis of about 1 micron, the disclosed droplets in the reflective PDCLC displays have a major axis that is greater than the cell gap (i.e., the thickness of the layer containing the polymerized liquid crystal). In FIG. 5, the cell gap is shown as 4 μm. The patent teaches that by having the droplet size much larger than the pitch of the cholesteric liquid crystals, inside the droplets the liquid crystal molecules behave similar to surface modified reflective cholesteric devices.
In U.S. Pat. No. 6,556,262 a reflective PDCLC with memory (i.e., bistable) is disclosed having a preferred droplet range of 8-10 micron. The focal conic state is so weakly scattering as to be described as being transparent. In the later US Application 20060066803 by the same applicant, it is stated that the contrast of a PDCLC display is degraded if there is more than a single layer of droplets sandwiched between the electrodes at most points of the display. It is further stated that preferably the droplets have a ratio thickness:length from 1:2 to 1:6.
Reflective, bistable PDCLC devices can be prepared by adopting the methods used in PDLC and NCAP devices. In the documents already cited, U.S. Pat. No. 6,061,107 uses the method known as Thermally Induced Phase Separation TIPS to prepare the PDCLC disclosed; U.S. Pat. No. 6,556,262 uses the emulsification method (also used by NCAP devices); and in the article by Tod Schneider et al. in SID 05 Digest, pages 1568-1571, a photoradical Polymerization Induced Phase Separation PIPS method is described.
Fast-Switching, Low-Voltage Devices
It is known that liquid crystal devices containing polymer walls or polymer networks (e.g., PSCT devices) have faster switching times (i.e. turn ON and OFF) and lower operating voltage than devices without such walls or networks. But PDLC displays are an exception in that the small droplet size typical of such films is known to increase switching times and operating voltage.
U.S. Pat. No. 6,203,723 discloses a PDLC type film having microencapsulated droplets that contain not just nematic liquid crystal, but also polymer network (similar to PSCT devices). The polymer network disrupts the alignment of the nematic liquid crystal within droplets causing domains to form therein, each domain has a different liquid crystal molecular alignment, and the polymer network stabilizes the alignment. While such devices have improved switching characteristics, and light scattering, the polymer network within droplets will cause increased haze as the interface surface area between polymer and liquid crystal is significantly increased. Light refracts not just at a droplet's polymer surface, but also as it enters and leaves the dense polymer network within a droplet. Such devices are unsuitable for the applications contemplated herein because of their high level of haze in the ON state.
U.S. Pat. No. 5,455,083 discloses a cholesteric liquid crystal optical shutter for projection type applications having superior operating voltage without loss of switching speed. The polymer is said to be formed into a structure of thin cell walls, and the introduction of these cell walls is said to create more focal conic domains per volume when compared with a continuous cholesteric liquid crystal layer that does not contain any polymer. The scattering properties of the disclosed device are not compared with a conventional PDLC film, and it is likely that scattering occurs principally at the interface between a polymer wall and the liquid crystal—similar to PDLC devices. There is nothing in the document to show that the scattering within droplets, at the boundaries of polydomains, is anything other than weak, and the latter scattering mechanism is not discussed in the document. In the patent's examples the OFF state is only required to scatter a 2 mm parallel light beam by plus or minus 0.57 degrees (2 mm aperture at 100 mm from the device) for it not to be received by the sensor. Clearly, the light scattering available from such devices is not sufficient to block visual access in films used for glazing applications.
Furthermore, the patent's examples 1 and 2 show that a film with 9% polymer content has good light transmission normal to the device's surface—90%—but that this falls to 75% when the polymer content is 15%—example 3. The reduced light transmission in the latter example is caused by light scattering (i.e., haze) in the ON state. This shows that such devices have significant haze and are unsuitable to meet the haze-free viewing requirements of the applications contemplated herein.
Furthermore, if the polymer content is to be held at 9% to avoid significant haze in the ON state, then a film with such a polymer content is unlikely to possess sufficient strength to be compatible with glass lamination processes, or use in large (<1M2) glass laminates.
Lastly, though the document claims to form droplets of liquid crystal within the polymer composite film, at a polymer content of 9% there is insufficient polymer to form discrete droplets in the range indicated, rather it is more likely that an interpenetrating network of liquid crystal volumes having thin polymer walls is formed. The type of polymer system used further supports this contention: oligomers of urethane acrylate and a high percentage (20% by weight of prepolymer) of trifunctional acrylate trimethylol propanetriacrylate monomer. In the previously cited U.S. Pat. No. 6,203,723 such a polymer system at a polymer content of 6% was used to create a “web-like structure (polymer network) within liquid crystal”.
In summary, devices of the type disclosed in U.S. Pat. No. 5,455,083 are unsuitable for the glazing applications contemplated herein because they have insufficient scattering power in the OFF state, suffer from too much haze in the ON state, have insufficient structural strength at the polymer content disclosed, and lastly, the liquid crystal/polymer composite film is not self sealing as discrete droplets of liquid crystal are not formed.
One of the inventors of U.S. Pat. No. 5,455,083 is also the inventor of a number of related devices having improved switching characteristics in common, for example: US 2004/0017523, U.S. Pat. No. 6,924,873, WO 01/55782 and WO 02/093241. Similar to U.S. Pat. No. 5,455,083, the polymer content is 10% or less and the polymer system is also similar. The liquid crystal device in US 2004/0017523 for example is said to have the liquid crystal/polymer composite film of the type in JP4119320. The polymer network in the latter is described as “a three dimensional mesh shape” and is shown in the document's drawings as allowing interpenetrating regions or volumes of liquid crystal. In addition, nematic liquid crystal molecules are shown aligning parallel to each other and perpendicular to the local polymer wall surface. For the reasons cited previously for U.S. Pat. No. 5,455,083, these devices are not suited for the applications contemplated herein.
U.S. Pat. No. 5,559,615 envisages a PDLC device for use in an active matrix type display where substituting the prior art nematic liquid crystal with a cholesteric type liquid crystal will improve such a device's turn-off time and light scattering. On turning off the electrical field the “(chiral) twisting force strongly acts between the liquid crystal molecules. For this reason the aligned state (ON) of the liquid crystal molecules is quickly returned to a twisted/aligned state.” The light scattering power will be increased “Since liquid crystal molecules are set in a twisted/aligned state in the absence of an electric field, the randomness (degree) of alignment of liquid crystal molecules is high, and the difference between the refractive indexes of the polymer resin and liquid crystal constituting the polymer dispersed liquid crystal film is large.” Crucially, the document sees the scattering mechanism as being refraction of light at the liquid crystal/polymer interface. Nowhere in the document is it envisaged that light scattering will occur at the boundaries between liquid crystal domains within droplets.
The document envisages using the same polymer system as in prior art PDLC devices, specifically that shown in the document's prior art FIGS. 13A and 13B. In FIG. 13A nematic liquid crystal molecules adjacent the polymer surface are shown aligned parallel to the local surface. Other than stating that the polymer system of prior art PDLC devices will be used, and discussing in general a photo-radical polymerization induced phase separation type process, the document does not show how to form the polymer structure, or what the characteristics of that structure might be. For example, the following are unknown: the prepolymer components, the percentage weight of each component, the percentage weight of polymer in the liquid crystal mixture, suitable types of nematic liquid crystal and chiral dopants, or UV curing conditions. It is clear from the preceding that the applicants envisaged a device having the same polymer structure in all respects as prior art PDLC devices. It follows from this that the liquid crystal droplets were also envisaged to be the same as in PDLC devices—0.7 micron to 1.0 micron mean diameter. Furthermore, the document's FIG. 4A shows that in the document the mean diameter “d” of a droplet is the same as “the mean diameter of the liquid crystal domains”, and, the only requirement is that “d” is larger than the helical pitch. Again, this requirement is satisfied by the prior art PDLC droplet size: 0.7 to 1 micron.
But, the present applicant has found that prior art PDLC droplet sizes of 0.7 micron to 1.0 micron are too small to achieve significant light scattering within droplets, regardless of whether nematic or cholesteric liquid crystal is used. The main scattering mechanism in such devices remains a droplet's polymer/liquid crystal interface, and so devices of the type envisaged by U.S. Pat. No. 5,559,615 have a different principle scattering mechanism, and significantly weaker scattering power, than the devices of the present invention.
It is apparent from the preceding analysis that there is a need for a liquid crystal film that has the structural advantages of PDLC films in terms of compatibility with lamination to glass, but which overcomes the significant increase in haze with viewing angle inherent in PDLC films. PSCT films, while having superior optical properties in some respects, lack the internal structure of PDLC films and have been found to be not compatible with lamination to glass and other laminates using conventional means. PDCLC films in the prior art are not suited for use in glazing application as their optical properties limit their use to reflective and conventional displays.